The Stars of the Milky Way

Our galaxy, the Milky Way, contains maybe 400 billion stars (plus or
minus 200 billion) that lie mostly in a flattened spiral disk of some
70-100,000 light-years (ly) across, with a
central bulge of about
10,000 ly in diameter. The sun, Sol, lies less than half way out
(26,000 ly) from the galactic center in Sagittarius, on the core-ward
side of one of the galaxy's spiral arms named after Orion that is
some 2-3,000 ly thick. Roughly 6,000 ly separate the Orion arm from
the Sagittarius-Carina arm on the inside and the Perseus
arm on the outside. Sol is located 67 ly north of the galactic plane
within a roughly 200-ly wide band that is rich in gas, dust, and newborn
stars, particularly the associations of extremely bright, bluish, and massive
O and B stars and emission nebulae (H II) that light up and define the spiral
arms. The apparent voids between spiral arms are actually full of dimmer,
redder, and less massive stars like Sol. From our perspective, the galactic
rim is in the direction of Auriga and Taurus.

The Milky Way has two
major and two minor arms
that spiral out from the
long bar of stars in the central bulge, as well as
near and far arms that
lie along both sides of
the central bar
(more).

Looking in from outside the Milky Way, we would see the luminous parts of the
galaxy. A dense concentration of mostly old stars fills the
central bulge,
whose brightest stars are red giants of relatively low mass and big bluish
stars recently born from gas held tightly around the galactic center (towards
an object called Sagittarius A* that may be a black hole massing about 2.5
million suns). The central bulge actually extends in a 12-18,000 ly long
"bar" (2-3 times longer than it is wide) to two to four bluish spiral arms on
opposite sides that wrap around the bulge and each other outwards through the
dimmer and redder galactic disk, possibly including broken arm segments,
yellowish "ghost" arms where most short-lived OB stars have already
perished, and spurs off the arms (for example, the Orion "arm"
containing Sol may actually be a spur of the Perseus arm). Surrounding the
Milky Way's spiral disk and bulge is the
slightly flattened galactic halo of
old stars, averaging somewhat lower in mass than our sun, including a
relatively small number of individual stars and 200 or so globular clusters
-- roughly half above and half below the disk.

Our luminous galaxy, however, appears to be embedded in a larger and much
more massive, but unseen,
spheroidal halo of nonluminous
material. The Milky
Way may actually contain as much as the mass of a trillion suns like Sol,
although the 400 billion, estimated luminous stars mass only about 175 billion
suns. Thus, most of the galaxy's mass must be composed of "dark" matter, of
which brown dwarfs, neutron stars, black holes, gas, and dust are estimated
to make up only a minor share. The nature of the galaxy's "missing"
nonluminous matter is still unknown.

Although as many as 5,800 to 8,000 of the Milky Way's stars are visible from
Earth with the naked eye, it is seldom possible to see more than 2,500 stars
at any given time from any given spot. Most of the "bright" stars
that we do see are atypical, with more mass, a higher luminosity, and a greater
diameter than our own sun. On the other hand, the vast majority of stars in
Sol's neighborhood are dimmer than Sol, too dim to be observed with the naked
eye. Although dim and reddish M-dwarf stars constitute more than half
of the population of our galaxy's stars, none are visible to the naked eye.

The Milky Way, however, appears to be poised to enter middle age. While
many younger galaxies can be grouped into "blue galaxies because their
vigorous star formation produces a lot of young stars that are massive, bright,
and bluish, older galaxies can be grouped into "red galaxies" where most
short-lived, bluish stars have expired to leave redder, dimmer, less massive
stars behind. Recent observations using infrared wavelengths indicate that
the Milky Way appears to be of intermediate color, which can be grouped into
relatively rare "green valley" galaxies, that are thought to be changing from
blue to red as star-formation waning over 1.5 billion years (based on model
simulations). If observations and model simulations are correct, star
formation in the Milky Way will end within five billion years, despite the
gas compression and star formation impact of a predicted merger with the
neighboring
Andromeda
Galaxy (M31) beginning in 2.2 billion years and taking five billion
years, which also appears to be transitioning into a green valley galaxy
that is relativley poor in the cold gas needed for star formation
(Mutch et al, 2011; and
David Shiga, New Scientist, May 20, 2011).

NOMENCLATURE. Many bright stars have proper names. Most are Arabic names
based upon the position of the star within the original Greek constellations
-- which we refer to using Roman translations of the Greek names. Many bright
stars lying within constellations have Bayer designations using the Greek
alphabet (from Alpha to Omega, generally by decreasing brightness) and/or a
Flamsteed number (beginning with 1, based on position in its constellation from
left to right) and the genitive spelling of the Latin constellation name.
(For example, the star "Keid" in Eridanus is more commonly known in
the United States as Omicron2 or 40 Eridani A, where the number 2 indicates
that there is another star in Eridanus designated Omicron1 and where the
following letter "A" designates this star as the "primary" or most luminous
star of a multiple star system.) In addition, many variable stars are also
designated with capital letters (e.g., R or RR) in front of the Latin
genitive for the nearest constellation.

Stars are also designated with catalogue numbers. While some begin with the
cataloguer's last name (e.g., Ross or Wolf), most use just the first
letter of the name(s), or some other abbreviation, to save space. Examples
include:

SPECTRAL TYPE. Astronomers differentiate the stars by spectral type,
a system of classification which indicates the star's predominant
color, a reflection of its surface temperature. The sequence of the
seven basic spectral types is denoted in capital letters, which have
been translated visually by ChView into colors that exaggerate their
actual tint, as derived from surface temperature, particularly at the
extremes of the spectrum, in the bluish and reddish tints of the
hottest and coolest stars, respectively. [New default colors can be
selected -- or created -- for ChView display.]

O

bluish-violet

Iota & Zeta (Alnitak) Orionis Aa

B

blue

Algol A, Regulus Aa

A

cyan

Sirius A, Vega

F

pale yellow

Procyon A, Eta Cassiopeia A (F9-G0)

G

yellow

Sun, Alpha Centauri A

K

orange

Alpha Centauri B, Epsilon Eridani

M

red

Proxima Centauri, Barnard's Star

Each spectral type is further subdivided into 10 divisions from 0 through 9,
hottest to coolest. Most stars are of type M, with diminishing numbers up to
type O -- quite rare in our galaxy. About 90 percent of all stars are main
sequence dwarfs of spectral type F through M (excluding 9 percent white dwarfs,
0.5 percent red giants, and 0.5 percent everything else). On the other hand,
the average mass of main sequence dwarf stars rises dramatically from M to O.

The current version of CHVIEW consolidates stars of unknown spectral type as
well as those of all other spectral types into "X". The non-OBAFGKM
spectral types include reddish giants and supergiants that have become
relatively rich in carbon, such as C (formerly R and N) and S types, as they
have run out of hydrogen as the primary fuel for nuclear fusion -- none are
currently believed to be located within 250 ly from Earth. WC (carbon-rich)
and WN (nitrogen-rich) Wolf-Rayet stars such as Suhail (WC8, Gamma2 Velorum Aa)
are massive stars (averaging 20 solar masses in binaries with O stars) that may
have already expelled 40 percent or more of their original mass, including
their entire hydrogen envelope. The extreme luminosity of Wolf-Rayets is
obscured by the dust and gas shed by them.

Analysis of the spectral lines found in star light yields additional
information, which is noted in lower case letters following the capital letter
denoting spectral type. Examples include:

e

emission lines

k

interstellar lines (chemical compounds in interstellar dust)

m

metallic lines

n

nebulous lines (e.g., rapid rotation)

p,pec

peculiar lines

s

sharp lines

v,var

variable lines

w

weak lines

Some stars have stellar companions so close that they appear to be single
stars. Analysis of the spectra from such stars may indicate Doppler shifts
from the movements of the stellar pair, suggesting the presence of a companion
star. Called spectroscopic doubles (spec.dou.) or binaries (SB), spectral
lines found in their star light may be periodically doubled
("double-line" binary). If one star's spectrum is too faint to be
seen, however, the spectral lines of the primary star may oscillate about a
mean position ("single-line" binary). In astrometic binaries, the
presence of an invisible companion is inferred from slight
"wobblings" in the motion of the primary.

MASS & EVOLUTION. A star is probably born as a nebular cloud of gas and
dust of interstellar size collapses, spinning inward via an accretion
disk towards an increasingly dense core. While still obscured from view
by dust, nuclear fusion may ignite at the center of these pre-stellar
objects, as hydrogen is fused into helium. Proto-stars (pre-"main sequence,"
T-Tauri stars and Herbig-Haro objects, possibly R Coronae Australis/CD-37 13027
which may only be 27 ly from Sol) are born as the energy of hydrogen fusion
pushes outward to balance the inward pull of gravity. Eventually, any
surrounding -- possibly stellar amounts of -- gas and dust that remain around the
star will be blown away by the star's radiation in T-Tauri winds, including much
that may be infalling through its stellar disk and blown out in jets (that may be
driven by an intense magnetic field) at the star's poles. Once fusion begins,
planets may have only a few hundred thousand years or so to form from
proto-planetary objects before the dusty circumstellar disk becomes too tenuous,
and the star enters the main sequence. Young stars in the solar neighborhood
include many OBA type stars such as Beta Pictoris (A3-5), which is young
enough to have an easily detectable dust disk, Epsilon Eridani (a K2V with a
dust ring as wide as 60 times the Earth to sun distance that is estimated to
be between one half and one billion years old), the double-binary system of
HD 98800 which has four main-sequence K and M stars that may be only 10
million years old.

Astrophysicists have concluded that, in order to sustain the nuclear reactions
necessary to become a star, a gaseous body must have about 0.074-0.080 the mass of
our sun. Currently, some astronomers have been defining smaller objects with
between 0.013 and 0.080 solar masses (about 13 to 80 Jupiter masses) as
"brown" dwarfs, especially those in particularly eccentric orbits
around a star since such objects probably formed at about the same time as
the star from the same nebular cloud. (According to at least one theory,
a "super planet" that is detected in a close-in orbit could be either
a brown dwarf or a gas giant that formed after the birth of the star from a
dust disk but migrated inward as the result of friction with dust or some
other mechanism.) While at least one theory predicts a huge number of brown
dwarfs, their dimness make them difficult to detect and few discoveries were
actually confirmed until recently. In 1995, however, a brown dwarf companion
to Gliese 229 (only about 19 ly distant) was not only confirmed by the
international astronomical community but also photographed by the Hubble Space
Telescope; by late 1996, at least 12 were found within 250 ly of Earth --
even more if farther objects are counted.

Most stars begin their life as "dwarfs" and spend the bulk of their
lifetime in the main sequence with the same spectral type. As a star ages
within the main sequence, however, it can become more as well as less luminous.
Once a star moves off the main sequence, it will eventually swell so large
that it's surface temperature will drop enough to shift its spectra
dramatically downwards. Some large stars will shift spectral class down and
back up more than once, as they shed mass through stellar winds and outbursts
like nebulae and novae and shift from core hydrogen fusion (e.g., to helium,
carbon, then oxygen) as their primary source of radiant energy.

Roughly 90 percent of all dwarf stars have a mass between 0.085 (M8) and 0.8
(G8) in theory -- such as Tau Ceti (G8p) -- of that of our sun, Sol. About
10 percent of the stars, including Sol (G2), lie within the intermediate main
sequence. While the lower limit of this range is 0.9 solar masses, the upper
mass limit is uncertain, somewhere between six (B5) and 10 (B2) solar masses
in theory.

Most of the stars discussed thus far will swell up and become giant stars for a
period of about 20 percent of their main-sequence lifetime, as they use up
their core hydrogen and begin fusing helium then heavier elements at higher
temperatures. Our own sun will leave the main sequence by expanding from its
current diameter of about 0.01 of an astronomical unit (AU) -- 1/100 of the
distance from the Earth and to the sun -- to as much as one AU. Eventually,
these giant stars will literally puff off their cooler outer layer of mostly
unfused hydrogen into interstellar space as planetary nebulae, leaving behind
the dense cores called "white" dwarfs that are as small as, or
smaller, than the Earth in diameter but with 0.5 to 1.4 solar masses.

Less than one percent of all stars lie in the upper main sequence, between
about eight (B3) and 120 (O3) solar masses in theory. These stars quickly
consume their core hydrogen, swell up into larger "supergiants,"
but may blow up in supernovae. The end result of such explosions may be a
neutron star (1.4 to six solar masses) or a more massive a black hole.

Small, cool, and faint M stars -- such as nearby Barnard's Star (M3.8)
which may already be around 10 billion years old -- may last for 50 billion
years or more before cooling into black dwarfs. However, the more massive a
star is, the faster it consumes and sheds its mass, and the shorter it
"lives" as a star. [The lifespan of a star is a function of their
mass (or energy supply) and luminosity (rate of energy consumption) -- roughly
proportional to 1/Mass raised to the power of 2.5.] The hottest and most massive
stars may use up their hydrogen at such a pace that they last less than a
million years, short compared with our sun's expected lifetime of about 10
billion years -- still about five billion to go. Vega, type A0, may live only
one billion years; a type B star may live for 30-some million years; and the
massive O types may last only as long as three to four million years. Hence,
the OBA stars observed in our skies are relatively young stars compared with
redder spectral types, and any planets found around these stars are unlikely
to have had the time to have evolved multi-cellular lifeforms similar to those
found on our 4.5 billion-year-old Earth. Since the estimated age of our
galaxy is about 13 billion years or less, none of the lower mass stars (M to
G8) have had time to fade from view, but most of the previously born, higher
mass stars (B to O) have already perished.

Stars also are assigned luminosity classes:

0

Hypergiants

V810 or Omicron1 Centauri (?)

Ia,b

Supergiants

Antares Aa, Canopus

IIa,b

Bright Giants

Dubhe A, Tarazed

IIIa,b

Giants

Aldebaran Aa, Arcturus

IVa,b

Subgiants

Procyon A, Beta Hydri

Va,b

Main Sequence Dwarfs

Sol, Sirius A

VI/sd

Subdwarfs

Kapteyn's Star

VII/D

"White" Dwarfs

Procyon B, Sirius B

This classification system is perhaps a better indicator of a star's relative
age and stage of evolution within its class as well as of its mass. Subdwarfs,
such as nearby Kapteyn's Star (M0VI or M0sd), are more bluish than younger
main-sequence dwarf stars and have a lower "metals" content
of elements heavier than helium -- perhaps due to their birth in an earlier
age (or region) of the galaxy when relatively few supernovae had as yet
spewed their metals into surrounding dust clouds. Most of the stars in
the central bulge and in the globular clusters of the galactic halo are
old, low metals stars.

LOCAL STARS & STELLAR POPULATIONS. Stars in the solar neighborhood
include representatives of the two major stellar populations of the galaxy,
disk and halo stars.

Including the stars of the distant globular clusters, halo stars are among
the galaxy's oldest, thought to be mostly 10 billion years and older. While
halo stars are only very weakly concentrated towards the galactic plane, they
exhibit a strong concentration towards and including the galactic nucleus but
with highly eccentric orbits. These stars contain a very low metals abundance
relative to the sun (with a mean around 0.02 of Sol's). Not surprisingly,
there are very few halo stars are in the solar neighborhood (perhaps as
low as 0.1 percent), but they include local subdwarfs, Kapteyn's Star (M0VI or M0sd)
and Groombridge 1830 (a G8VIp with "superflares" that is now believe to be a
single star -- no M-type flare star companion). Also called
Population II stars because of their later discovery, this group also includes
RR Lyrae variables with periods greater than 12 hours, subdwarfs and other
extremely metal-poor stars, and some red giants.

Often called Population I stars, the relatively younger stars of the galactic
disk can be further subdivided into four distinctive groups: very young spiral
arm; young thin disk; intermediate-age disk; and older thick disk and nucleus.
As mentioned previously, the spiral arms include most of the galaxy's
interstellar gas and dust, young stars, and stellar associations, including:
O and B stars; supergiants; Cepheid variables; pre-main sequence, T-Tauri
stars and Herbig-Haro objects (e.g., R Coronae Australis/CD-37 13027); and
some A stars. Less than a hundred million years old, these stars are
rich in metals (as rich as, but ranging up to twice, Sol's abundance) and have
highly circular galactic orbits within 1,000 ly of the galactic plane. While
often extremely bright when not obscured by dust, these stars probably total
substantially less than one percent of all Milky Way or nearby stars.

Spiral galaxies like the Milky Way and its largest neighbor, Andromeda,
have large central bulges of mostly older stars,
as well as a relatively young thin spiral disk (surrounded
by older, thick disk stars that may have come from mergers
with satellite galaxies) and a
luminous halo that includes
numerous globular clusters
(more).

Young thin disk stars lie within 1,500 ly of the galactic plane and have galactic
orbits of low eccentricity. Around one billion years or more in age, they
include many A and F stars, AFGK giants, some GKM main-sequence dwarfs, and
white dwarfs. While they have a mean metals abundance near Sol's (1.0),
some may be twice as rich. Totalling as much as nine percent of all stars in
the solar neighborhood, they include Sirius2 (A0-1Vm and A2-5VII -- also DA2-5)
and Vega (A0Va).

Intermediate-age disk stars include our Sun (G2V), most G and some K and M dwarfs,
some subgiants and red giants, and planetary nebulae. Many are around five
billion years old and have a metals content ranging from 0.5 to 1.0 of Sol's
(with a mean around 0.8). These stars lie within 3,000 ly of the galactic
plane, with moderately eccentric galactic orbits. For example, Sol is
traveling at seven kilometers per second northward out of the plane and may
eventually rise 200-250 ly above it after 15 million years, while the
Alpha Centauri3 (G2V, K1V, and M5.5Ve flare star) system may eventually travel
about 800 ly out with an upward velocity that is three times faster. As much
as 84 percent of the stars in the solar neighborhood are included in this
group.

Most thick disk and many nucleus stars are old. While many are more than eight
billion years old, they are probably less than 10 billion
years old. They include many K and M dwarfs, white dwarfs, some subgiants and
red giants, moderately metals-poor stars, long-period variables, and RR Lyrae
variables with periods less than 12 hours. Most thick disk stars lie within
5,000 ly of the galactic plane (thick disk mean of 3,500 ly) and have
considerably eccentric orbits. Their metals abundance ranges from 0.2 to 0.5
of Sol's (with a thick disk mean of 0.3). Thick disk stars may comprise as
much as four percent of nearby stars, including Lalande 2115 (M2.1V) which is moving
perpendicular to the galactic plane at a fast velocity of 47 km/sec.

NOTABLE NEARBY STARS. More information on specific nearby stars (with
links to research papers abstracted and electronically scanned by NASA
and other organizations or by individual astronomers) is available at
SolStation.com's web pages on
Notable Nearby Stars.